This read-only PerfCounter64 counts Bytes Tail Dropped by this network port's QoS shaper during the interval, meaning bytes discarded because the network-facing port's output queue was completely full. A rising count tells an administrator that traffic aggregated from multiple customers onto this uplink is overwhelming the port's shaped buffer. For example, a spike in f3NetPortQosShaperStatsBTD during a peak-hour window would suggest the uplink itself, not any single customer port, needs additional capacity.